The present disclosure relates generally to vehicle energy storage systems and, more particularly, to vehicle storage system control methods and a method for characterizing traction battery energy and power performance and project remaining service cycle life for heavy-duty hybrid electric vehicle applications.
In electric vehicles and hybrid electric vehicles (e.g., locomotives, off-highway mining vehicles, buses and automobiles), it is necessary to control the operation of the energy storage system in order to obtain high mission performance in terms of average mission speed, range, and/or payload capability, as well as to maximize the operating life of the energy storage system (ESS) and to avoid prematurely degrading thereof. For hybrid vehicles, it is also desirable to maximize the benefits of fuel and/or emissions savings. Existing energy storage systems in such vehicles may include one or more types of batteries, ultra-capacitors and/or flywheel systems.
ESS power command has traditionally been determined based on current drive power requirements, the ESS state of charge (SOC) or stored energy, and static ESS terminal power limits. The power sharing between individual banks in an ESS has further been based on the bank's SOC or stored energy, usable or rated energy capacity, and/or power limits. However, as between one or more individual energy storage banks, there may be a variation in the SOC that, utilizing conventional ESS power commands, could result in premature degradation of the ESS. Thus, it is desirable to be able to obtain greater life/less degradation of the energy storage system.
The performance characteristics for batteries used in electric vehicles and hybrid electric vehicles are normally specified by the manufacturer based on the specific energy (Wh/kg) thereof, volumetric energy density (Wh/l) thereof, and specific power (W/kg) thereof. In particular, the specific power characteristic is based on a “matched impedance” technique, wherein maximum power is transferred from the battery to the load (i.e., half of the power is dissipated in the load, while half of the power is dissipated in the battery's internal resistance). While this approach is useful in comparing one battery to another battery, it is generally not a good indication of the performance in an electric vehicle or hybrid electric vehicle, since the voltage where maximum power is transferred is 50% of the open circuit voltage.
Moreover, the energy rating of the battery is typically the total energy stored in the battery, not the useable energy. In an electric vehicle application, the lower limit for the SOC is typically somewhere around 20% of the total charge, or stated another way, around 80% of the Depth of Discharge (DOD) of the battery. Thus in the electric vehicle application, the useable energy is typically around 80% of the battery's total energy. Accordingly, the battery cycle life for an electric vehicle battery is often reported to be a number of 0–80% DOD cycles, after which point the available battery energy is reduced by 20% from the battery's original energy rating. Accordingly, at the battery's end of life, the electric vehicle will experience a 20% decrease in range.
In contrast, batteries for hybrid vehicle applications are typically operated over a significantly smaller range of DOD's as compared with an electric vehicle. As such, the useable energy of the hybrid vehicle battery is significantly lower than 80% of the battery's energy rating (as is the case for an electric vehicle). However, in the hybrid electric vehicle application, power is of particular concern, and therefore the battery's performance and life cycle should address both the discharge as well as the charge power levels. During vehicle deceleration or while holding speed on a down hill grade, the battery is expected to absorb high power levels. This condition is often referred to as regenerative braking. In small hybrid electric vehicles (e.g., passenger cars and vans), the regenerative braking interval is usually on the order of a few seconds; however, for heavy duty hybrid electric vehicle applications, the regenerative braking periods are on the order of 10s to 100s of seconds in duration or longer. As such, an improved method of battery characterization and determining battery life projection is also desirable.